The current development towards truly mobile computing and networking has brought on the evolvement of various access technologies that also provide the users with access to the Internet when they are outside their own home network. At present, wireless Internet access is typically based on either short-range wireless systems or mobile networks, or both.
Short-range wireless systems have a typical range of one hundred meters or less. They often combine with systems wired to the Internet to provide communication over long distances. The category of short-range wireless systems includes wireless personal area networks (PANs) and wireless local area networks (WLANs). They have the common feature of operating in unlicensed portions of the radio spectrum, usually either in the 2.4 GHz Industrial, Scientific, and Medical (ISM) band or in the 5 GHz unlicensed band.
Wireless personal area networks use low cost, low power wireless devices that have a typical range of about ten meters. The best-known example of wireless personal area network technology is Bluetooth, which uses the 2.4 GHz ISM band. It provides a peak air link speed of one Mbps, and power consumption low enough for use in personal, portable electronics such as PDAs and mobile phones. Wireless local area networks generally operate at higher peak speeds of 10 to 100 Mbps and have a longer range, which requires greater power consumption.
Wireless LAN systems are typically extensions of a wired network, providing mobile users with wireless access to the wired network. Examples of wireless local area network technology include the IEEE 802.11a, which is designed for the 5 GHz unlicensed band, and uses orthogonal frequency division multiplexing (OFDM) to deliver up to 54 Mbps data rates; the 802.11b, which is designed for the 2.4 GHz ISM band and uses direct sequence spread spectrum (DSSS) to deliver up to 11 Mbps data rates; and the HIPERLAN Standard, which is designed to operate in the 5 GHz unlicensed band.
In wireless LAN technology, two basic network topologies are available for network configuration: an ad-hoc network and an infrastructure network. An ad-hoc network is formed by two or more independent mobile terminals without the services of a base station, i.e. in an ad-hoc network the terminals communicate on a peer-to-peer basis. An ad-hoc network is normally formed for temporary purposes. The infrastructure network, in turn, comprises one or more wireless base stations, called access points, which form part of the wired infrastructure. In a typical network of this type, all traffic goes through the access points, regardless of whether the traffic is between two terminals or a terminal and the wired network, i.e. the mobile terminals do not communicate on a peer-to-peer basis. The mobile terminals are provided with wireless LAN cards, whereby they can access the wired network or set up an ad-hoc network. In an infrastructure network an access point and at least one terminal is said to form a Basic Serving Set (BSS), while an ad-hoc network is also termed an Independent BSS (IBSS).
So far, wireless LAN technology has been used mainly in laptop computers, which are typically AC powered, but which may also be used in battery mode that provides a fairly high battery capacity. To prolong the life of the batteries, the WLAN standards define a specific power save mode into which the terminals may enter from an active mode in order to decrease their power consumption. In this mode the WLAN-specific power consumption is very low, but the terminals have to wake up (i.e. enter the active mode) periodically to receive regular beacon transmissions broadcast in the network. The beacon transmissions indicate, for example, whether there are incoming packets buffered for a terminal. If so, the terminal retrieves the packets, goes back to sleep, and wakes up again to listen to the next beacon transmission.
The current WLAN power management has been designed assuming that the terminal devices are laptop type computers featuring a relatively high battery capacity. Along with the generalization of various other types of personal communication devices, such as intelligent phones, having a smaller size and thus also a lower battery capacity than laptop computers, power consumption has, however, become a critical issue when new properties are designed for wireless systems and terminals. Emerging ad-hoc mode applications in which power consumption may be rather high further aggravate the problem. Examples of such applications are games played in small groups or business meetings in which large files may be shared (wirelessly) by the participants.
Power consumption of the WLAN terminals has normally been reduced by trying to maximize the time the terminals spend in the power save mode. However, this does not affect the power consumption in the active mode, whose proportion of overall time, i.e. percentage of time in active mode, is on the increase along the generalization of new functionalities.
The present invention seeks to accomplish a solution by means of which the power consumption of a terminal may be reduced in the active mode of the terminal in a WLAN-like communication system provided with an uncoordinated media access mechanism, i.e. in a system where the terminals have to sense the common air medium prior to transmission to see whether the medium is free.